Differential Responses to Salt Stress in Four White Clover Genotypes Associated With Root Growth, Endogenous Polyamines Metabolism, and Sodium/Potassium Accumulation and Transport

Selection and utilization of salt-tolerant crops are essential strategies for mitigating salinity damage to crop productivity with increasing soil salinization worldwide. This study was conducted to identify salt-tolerant white clover (Trifolium repens) genotypes among 37 materials based on a comprehensive evaluation of five physiological parameters, namely, chlorophyll (Chl) content, photochemical efficiency of PS II (Fv/Fm), performance index on an absorption basis (PIABS), and leaf relative water content (RWC), and to further analyze the potential mechanism of salt tolerance associated with changes in growth, photosynthetic performance, endogenous polyamine metabolism, and Na(+)/K(+) uptake and transport. The results showed that significant variations in salt tolerance were identified among 37 genotypes, as PI237292 and Tr005 were the top two genotypes with the highest salt tolerance, and PI251432 and Korla were the most salt-sensitive genotypes compared to other materials. The salt-tolerant PI237292 and Tr005 not only maintained significantly lower EL but also showed significantly better photosynthetic performance, higher leaf RWC, underground dry weight, and the root to shoot ratio than the salt-sensitive PI251432 and Korla under salt stress. Increases in endogenous PAs, putrescine (Put), and spermidine (Spd) contents could be key adaptive responses to salt stress in the PI237292 and the Tr005 through upregulating genes encoding Put and Spd biosynthesis (NCA, ADC, SAMDC, and SPDS2). For Na(+) and K(+) accumulation and transport, higher salt tolerance of the PI237292 could be associated with the maintenance of Na(+) and Ca(+) homeostasis associated with upregulations of NCLX and BTB/POZ. The K(+) homeostasis-related genes (KEA2, HAK25, SKOR, POT2/8/11, TPK3/5, and AKT1/5) are differentially expressed among four genotypes under salt stress. However, the K(+) level and K(+)/Na(+) ratio were not completely consistent with the salt tolerance of the four genotypes. The regulatory function of these differentially expressed genes (DEGs) on salt tolerance in the white clover and other leguminous plants needs to be investigated further. The current findings also provide basic genotypes for molecular-based breeding for salt tolerance in white clover species.